CHAPTER SIX 



ACTION OF ULTRAVIOLET LIGHT ON VIRUSES 



The biological action of ultraviolet light has been the subject 

 of much study. The side of this work which is of most interest 

 here is ably summarized by McLaren (1949). An important 

 pioneer in the field of ultraviolet effects on viruses and enzymes 

 was Gates (1930). The more recently available purified virus 

 preparations have enabled some important new contributions to 

 this field to be made and there is, therefore, considerable ma- 

 terial for this chapter. In its simplest terms, ultraviolet light 

 gives information about the absorption of ring aromatic com- 

 pounds, purines and pyrimidines, and the peptide bond. The 

 former are associated with proteins; the purines and pyrimi- 

 dines, with nucleic acids, and the peptide bond is related 

 again to proteins. So a separation of protein and nucleic-acid 

 absorption is possible, and this is one of the first functions of 

 ultraviolet studies. More refined studies can tell something of 

 the relative importance of aromatic amino acids in various 

 kinds of function. An additional feature of importance is the 

 quantum yield of a biological result, a number which can be 

 related to both structure and purpose. 



Molecular Absorption of Ultraviolet Light 



It is in order to consider first the mechanism of the absorp- 

 tion of ultraviolet light. Light is electromagnetic radiation and 

 so interacts with the electric and magnetic elements of molecules. 

 In particular, it does so with oscillating electric dipoles, magnetic 

 dipoles, and electric quadrupoles ; but in actual fact the degree 

 of interaction is only appreciable for electric dipoles. So, for all 

 intents and purposes, we can say that light is only absorbed by 



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